This invention relates to the release of pressure from oxidized ore slurry in an autoclave circuit. In particular, the invention relates to the design of a nozzle system through which ore slurry passes into slurry flash tanks.
Autoclave circuits are used to recover gold from refractory sulfidic ores. Ore leaving an autoclave is typically passed to a series of flash tanks where pressure is let down and steam is flashed off to cool the slurry, and reduce it to atmospheric pressure for further processing. Steam from each flash tank is recycled and contacted with autoclave feed slurry in a complementary splash condenser, operated at substantially the same pressure as the flash tank, for preheating the autoclave ore feed slurry. In one particular system the pressure from the autoclave slurry discharge is let down in two stages. In the first stage, pressure is let down from about 420 psig to about 120 psig. In the second stage, pressure is let down from about 120 psig to atmospheric. This second pressure drop corresponds to a much greater volume expansion than in the first stage.
Heretofore this second pressure drop from about 120 psig to atmospheric has been accomplished by use of a nozzle system comprising a straight tubular choke extending from outside the flash tank to inside the flash tank. The choke was surrounded by a ceramic lined blast tube extending further into to flash tank, as shown in FIG. 4. As the volume of the slurry expands rapidly upon passage through the choke, the blast tube was violently impacted with steam entrained with ore slurry. Catastrophic failure of the blast tube, resulting in ore slurry damaging and even breaching the low pressure flash tank, has occurred. The typical life of such nozzle systems, and in particular of the blast tubes, has been relatively short, for example, six weeks, depending on operating parameters, ore characteristics, and many other factors. Rebuilding and/or replacing such nozzle assemblies is expensive in terms of capital costs and in terms of process downtime.